badaxed,
Are you seeing trolling motor interference or are you seeing sonar-to-sonar interference or are you seeing both?
For the trolling motor interference you need to contact the Humminbird Customer resource folks again.
For sonar-to-sonar interference you need to try turning on and up the Noise Filter menu. If that does not help enough than your need to try selecting a Lower Range menu setting that is different than what the unit automatically selects. I think that the Lowrance unit has a ping rate adjustment or something that may help as well.
For the shallow water depth reading problem this could be tied in with either trolling motor interference or the sonar-to-sonar interference. If this is being caused by neither of these than try using both Beam select menu settings and see which give you the better readings.
Try adjusting the Surface Clutter menu.
As a last resort try setting the Water Type menu to the Salt (Deep) setting. Yes I know that sounds strange but it changes the algorithms used and may be a temporary fix that you can use.